The spines of many seniors are prominent because they have lost muscle tone. Tara, pictured above, is 16 years old and doing exercises to increase her flexibility. Regular fitness training has enabled her to use the stairs, wrestle with other dogs and play ball, as well as having very defined muscles. Shhhhh....She doesn't know that she is a senior.
Romeo, 6 years old, is using the peanut to strengthen his core, shoulders and hind quarters (in order to reduce the risk of injury while he is playing). Notice the muscle definition. I call him my body builder dog.
Dogs that have lost a limb need to relearn proprioception (get a new idea of where its body is in space) and how to balance. The remaining limbs need to become stronger (without over training) in order to be protected against undo stress on the limbs. Balance equipment is introduced and the exercises progressively get more difficult as the dog adapts.